Energy-efficient radio frequency (RF) power amplifiers (PA) are critical components in mobile, battery-operated wireless communication devices, e.g. mobile phones, personal digital assistants (PDA), etc., because they determine a significant portion of the total power consumption of such devices. Batteries with low supply voltage, typically about 3V, are employed in the portable devices. It has been possible to design a PA for the 3V battery voltage even if the efficiency already has dropped a bit. In order to get high efficiency from a PA, a switching-mode power supply (SMPS), often referred to as a Boost converter or a step-up converter is needed to up-convert the battery voltage to a value higher than the maximum battery voltage. With this approach, the needed output transmission power can be easily obtained from the PA, but the efficiency for small transmission power levels is poor. To improve efficiency also at the small transmission power levels, the supply voltage of the PA must be lowered. This is not possible with a Boost-type power supply but another type of switching-mode power supply (often referred to as a Buck converter, or a step-down converter) is provided to down-convert the raised battery voltage to the level needed for actual transmission power level. If the two converters are cascaded, the efficiency at low transmission power levels suffers from the cascaded converters and the high voltage between them.
Some RF PA applications with a switching-mode power supply (SMPS) use a dynamic biasing PA, such as the envelope elimination and restoration (EER) technique, to achieve high-efficiency, linear power amplification. In the EER, the phase and envelope information are extracted from the original modulated signal. A constant envelope signal containing the phase information is amplified using an RF PA. Since the phase information has a constant envelope, the PA can be highly overdriven to achieve high efficiency. The envelope information is fed into the power supply circuit to modulate the supply voltage of the RF PA (e.g. drain or collector voltage) and thereby superimpose the envelope variation. By changing the supply voltage, the output waveform will be shaped and the overall amplification can be linear. There are also simplified versions of the EER, such as the envelope tracking (ET) technique. In the ET technique, the input signal to the RF amplifier contains both the phase information and the amplitude information, and only the envelope information is extracted for the switching power supply. The RF PA is operated in the linear region and its supply voltage is changed according to the extracted envelope information. The supply voltage is varied with sufficient headroom to minimize distortion. Examples of transmitter architectures employing EER technique are disclosed in U.S. Pat. No. 6,256,482 and WO2006/085177. A still further version is power-level tracking (PT) in which a switching-mode power supply tracks only the slow-varying average power level, instead of the fast-varying envelope, and modulates the drain or collector voltage of a linear PA.
So-called Buck-Boost converters also exist. An example of a Buck-Boost converter is disclosed in U.S. Pat. No. 6,348,781. The Buck-Boost converters are designed to change mode from Buck to Boost automatically to provide a supply voltage that ensures good efficiency of an RF PA at any specific transmission power level. No cascading is needed. However, in applications that use the envelope elimination and restoration (EER) technique, a very tightly controlled frequency and group delay response is needed. With a Buck-Boost switching-mode power supply, the output voltage range where the mode changes is quite critical, and distortion occurs. Also, in the Boost mode of operation it is difficult to keep the response unaffected. Buck-Boost converters are good choices for power level tracking where the frequency and group delay requirements are loose.
The problems regarding efficiency are becoming more significant with the battery technology, e.g. lithium battery technology, lowering the cut of voltage from the present range, e.g. 3V, to about 2.5V or lower. As a consequence, a transmission (TX) power amplifier becomes very inefficient and variation in its performance increases, if it is designed for 2.3V supply voltage, for example.